To enhance the contrast and improve the brightness ratio of CRTs, manufacturers developed and commercially employed black matrix on the interior surface of faceplate panels of CRTs such as that described in U.S. Pat. No. 3,558,310, issued to Mayaud on Jan. 26, 1971. The matrix commonly consisted of graphite structures printed on panels whereby the structures have openings into which respective phosphor deposits are to be printed. In current entertainment CRTs, the matrix structures are vertical stripes of light-absorbing material, wherein the openings are the spaces between adjacent stripes.
In the 1970's, CRT faceplate panels were clear glass having a center transmission of about 84% and an edge transmission of about 80%. The lower edge transmission was due to the panel glass thickness being higher in the edge area than the center area. The panel thickness needed to be larger at the edge of the panel relative to the center in order to gain sufficient thermal, mechanical and vacuum strength to survive the subsequent processing of the CRT. Essentially, the required strength of the glass in the panel dictates the thickness of the panel at the center and edges and, in turn, the desired curvature of the panel dictates the required strength of the glass. In the early days, the panels had greater curvature in that the radii of the panel surfaces were relatively small. These products were referred to as 1R, where the 1R designation refers to a radius of curvature of the interior panel surface. The standard for the 1R designation is a 25V CRT having a radius of curvature on the interior of about 40.7 in., wherein “1” is a numerical coefficient and “R” is equal to 40.7 in. for 25V product. From this designation, one of skill in the art knows the approximate interior curvature of the 25V panel based on the numerical coefficient in front of the “R,” e.g., 2R would imply a radius of about 81.4 in. for a 25V panel. Those skilled in the art also may recognize that the value of “R” is approximately linearly proportional to the diagonal screen dimension. Therefore, one would expect that the value of “R” for a 35V product would be about 1.4 times greater than that for the “R” of a 25V product, because the diagonal of a 35V is about 1.4 times greater than that for the 25 V product.
In the mid 1980's and early 1990's, the glass transmission was reduced to nominal values of about 52% and 42% by adding the dark tint compounds in glass melt, which exaggerated center to edge glass transmission difference to a ratio of 1.4-1.5 to 1. Additionally during this timeframe, the CRT market was directed to improved cosmetic appearance and improved viewing angles; as such, the trend was to go to panels having greater radii, such as the so-called 1.5R and 2R product. In fact, this trend continues to today, wherein CRTs are being designed and built which are virtually flat. Each change toward flatter CRTs requires the panels to have relatively even greater thickness at the edges with respect to the center to gain adequate strength. The result, however, has been an even greater reduction in the corner or edge glass transmission ratio. For example, in a true flat CRT product where the center transmission is about 52%, the corner to center transmission ratio is about 0.5:1. This is undesirable because the edge or corner brightness of the CRT becomes much lower than the center brightness. Some CRT manufacturers have tried to compensate for the difference by reducing the center matrix openings. Unfortunately, such a reduction in matrix opening produces CRTs with an undesirable reduction in overall brightness and, in some cases, the contrast ratio.
To improve the brightness and the contrast ratio and prevent a large disparity between the center and edge transmission in CRTs having flat or nearly flat panel contours, some manufacturers have gone back to the clear glass having a central transmission of 84% and applied a neutral density laminate sheet on the exterior surface of panels of CRTs. The approach is adequate for center to edge brightness and overall brightness performance of the CRT, however, it is very costly. For example, the use of such a laminate increases the manufacturing cost for a 32V CRT by about 7 Euros. Therefore, the CRT industry is in need of a more cost effective means of manufacturing CRTs with flat or nearly flat panel contours that have acceptable overall brightness performance and contrast.